Method of configuring a ceramic core for casting a turbine blade

ABSTRACT

The internally air cooled turbine cast blade is molded with a ceramic core for defining the interior cavity with coolant passages where the ceramic core is configured to fair radially inwardly at the attachment portion of the blade until they form a single wall, parallel to and centered in the blade attachment. The blade is cast in the mold by pouring molten metal into the mold, curing and the core is leached once the metal is cured and the mold removed. Another embodiment includes a metering plate attached to the root of the blade for metering coolant to provide a coolant to gas flow ratio at the film cooling holes.

This is a divisional patent application of Ser. No. 08/689,983 filed onOct. 28, 1996, now U.S. Pat. No. 5,820,774.

This invention was made under a Government contract and the UnitedStates Government has an interest herein.

TECHNICAL FIELD

This invention relates to air cooled turbine blades for gas turbineengines and particularly to the casting configuration for molding theturbine blade and the method thereof.

BACKGROUND ART

As one skilled in the gas turbine engine's turbine blade technologyappreciates, the formation of the internal cooling passages in a turbineblade present formidable problems. Typically, these passages are formedin the molding process for casting the blade utilizing ceramic cores.These ceramic cores are extremely brittle and the thickness is finiteand obviously, have the propensity of breaking. Because of the materialand the finite sizes of these passages the producibility of the bladesare adversely impacted. Not only is the blade designer confronted withthese molding problems, he must also take into consideration theeffectiveness of cooling of the blades by virtue of these coolingpassages. This coupled with the fact that the sizes and locations of thepassages internally of the blade are critical, the cooling requirementsof the blades in order to maintain its structural integrity in theengine's hostile environment are severe compounds the complexity of thedesigner's task. In addition the designer must be cognizant of the factthat the air utilized for cooling is taken from the engine after acertain amount of processing of this air has occurred which obviouslyimpacts the performance of the engine. Not only is the amount of workthat has been done on that cooling air represents a penalty to theengine's performance, it is obviously necessary to utilize only thatmuch air that will handle the cooling load. Any extra air used for thispurpose accounts for an additional deficit in the performance of theengine. Hence, it is abundantly important that the blade designerdesigns the cooling aspects of the blade so that only the exact amountof cooling air is utilized over the operating envelope of the engine.Also, the designer must assure that the integrity of the blades is notcompromised while at the same time the efficiency of the engine is notjeopardized.

This invention solves the problem of effective blade cooling while atthe same time designing the core used in molding the blade such that theblade structure is enhanced while the breakage of the core used informing the cooling passages is eliminated or minimized. As is wellknown in this industry, core breakage has been a problem confronting theblade designer for some time. As will be described in more detailhereinbelow, by virtue of this invention both breakage and distortionsof the end part and the ceramic core assembly are eliminated orminimized. The core of the turbine blade is formed from a uniqueconfiguration which significantly enhances the producibility of both theend part and ceramic core that is utilized to make the end part. Theturbine blade fabricated from this invention involves a 4-walled,cooled, single-piece configuration and utilize a three piece assembledcore in which each piece is made using production-oriented methods andmaterials. The blade to which is being referred herein is not to bemistaken with the heretofore known multi-piece cores and 4-walledcastings.

In accordance with this invention, the turbine blade is formed from aunique configuration in which individual segments which make up theinner walls of the 4-walled casting taper and merge, alternating fromeither side of the airfoil while fairing radially inward, until theyline up to form a single wall, parallel to and centered in the bladeattachment portion. Essentially, this configuration effectively resultsin a 4-walled airfoil blending into a 3-walled attachment region witheach cavity element maintaining direct radial flow coolant feed.

This configuration for producing the ceramic cores necessary in makingthe hollow castings affords several advantages. For example, in a3-piece type core, where the ceramic core elements which produce thehybrid cavities are made separately from the main body ceramic coreelement, the present invention allows the hybrid cores on both sides ofthe main body core to be substantially thickened in the blade attachmentarea without excessively displacing the main body core. This is abenefit because the ceramic cores which produce the hybrid cavities arecantilevered, from the attachment area, for the entire length of theairfoil and would be too susceptible to breakage or deflection withoutthickening. Another benefit from thickening the hybrid cores in theattachment area is that coolant flow entry losses are minimized byhaving a larger feed area directly in line with the coolant flow.

The problem associated with the molding process is perhaps bestunderstood by referring to FIG. 1 which is a prior art illustration ofthe formation of the passageways in the turbine blade. As noted in FIG.1, the attachment portion of the blade depicted schematically isindicated by reference numeral 110. The inner passages formed by themulti-piece core is depicted by reference numeral 112 and location ofthe core joints are depicted by the dash lines 114. The disadvantage ofthis prior art configuration are numerous and include, withoutlimitation, the following:

a) these hybrid cores can not be thickened on both sides withoutdisplacing the main body;

b) On a 3-piece assembled core configuration, the core joints wouldrequire a matched plug and socket type of contour, increasing thepotential for mismatched surfaces;

4) Flash at the core joints would occur perpendicular to the coolantflow causing blockage of the cooling air;

5) Direct access for flash removal is difficult, if not impossible;Complete flash removal causes risk of damaging the surfaces of the bladeneck; and

6) There are no provisions for selectively metering coolant flow into aparticular hybrid cavity.

Not only does this invention afford advantages in producing the ceramiccores necessary in making the hollow casting, it affords advantages tothe cast part. As one skilled in this art appreciates, when amulti-piece ceramic core is used to produce a hollow casting, metalfinning or flash can occur at the joint lines of adjacent core elementswhen the gaps between the core element mating surfaces are notcompletely filled by the bonding material. This invention obviates orminimizes this problem because the core element joints are positioned ina location that is parallel to and centered in the blade attachmentportion therefore providing direct access for flash removal through theattachment coolant feed passages. Since any remaining flash or partiallyremoved flash will be parallel to the coolant flow stream, thisremaining flash will not interfere with the coolant flow stream andhence, flow blockage will not occur.

This invention also lends itself to attaching a metering plate at thefoot of the blade so that the cooling air can be selectively meteredinto a hybrid cavity. This is particularly important where it is desiredto improve the film cooling effectiveness across the outside wall of theblade. The metering plate allows the designer to size the inlet to theinternal cavities so that the pressure supplied therein is controlled soas to provide a desired pressure ratio between the coolant and theengine's working medium or gas path.

SUMMARY OF THE INVENTION

An object of this invention is to provide an improved internally cooledturbine blade.

An additional object is to provide for an internally cooled turbineblade means for metering the flow admitted internally of the blade.

A feature of this invention is the configuration of the core for formingthe cooling passages in the blade.

A further feature of this invention is the method of forming the coolingpassages of the blade with a ceramic core.

The foregoing and other features of the present invention will becomemore apparent from the following description and accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view down the blade attachment center line partially insection illustrating a prior art ceramic core;

FIG. 2 is a partial view through the same plane as the plane taken inFIG. 1 illustrating the present invention;

FIG. 3 is a sectional view taken through line 3--3 of FIG. 2;

FIG. 4 is a sectional view taken through line 4--4 of FIG. 2;

FIG. 5 is a sectional view taken through line 5--5 of FIG. 2:

FIG. 6 is a sectional view taken through line 6--6 of FIG. 2;

FIG. 7 is a sectional view taken through line 7--7 of FIG. 2;

FIG. 8 is a sectional view taken through line 8--8 of FIG. 2;

FIG. 9 is a sectional view taken through line 9--9 of FIG. 5;

FIG. 10 is a sectional view taken through line 10--10 of FIG. 5;

FIG. 11 is an enlarged, exploded and schematic view in perspectiveillustrating the core sections used in a typical casting made inaccordance with this invention;

FIG. 12 is a partial view of the mold for casting the blade including ametering plate attached to the root of the blade for metering coolantinto the internal passages of the blade; and

FIG. 13 is an elevated view of a typical cast prior art turbine blade.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference is now made to FIGS. 2-10 illustrating the details of thisinvention. As mentioned earlier the invention is concerned with a4-walled, cooled, single-piece cast turbine blade and teaches the uniqueconfiguration of the core which significantly enhances the producibilityof both the end part and the ceramic core assembly required to make it.The configuration utilizes either a three or four piece assembled coremade from a suitable ceramic material in which each piece is made usingproduction-oriented methods and materials.

Essentially, the pieces or individual segments of the core are uniquelyconfigured to make up the inner walls of the 4-walled casting. Thesegments taper and merge, alternating from either side of the airfoilwhile fairing radially inward, until they line up to form a single wall,parallel to and centered in the blade attachment. As will be appreciatedby those skilled in this art, this configuration effectively results ina 4-walled airfoil blending into a 3-wall attachment region with eachcavity element maintaining direct radial flow coolant feed.

As can be seen in FIG. 2 the core segments 10 are disposed in the cavity12 of the mold 14 as seen before being filled with molten metal andsubsequently leached. The dash lines 16 defines the dimension and shapeof the internal cavity of the blade adjacent the airfoil section 18(FIG. 13) and the portion below line 20 is the form of the fir treeattachment of the blade. The core segments as is typical in this type ofconstruction consists of the main body core elements, the pressure sidecore elements, and the suction side of core elements. The number of eachof these elements are dependent on the particular configuration of theinternal structure of the blade and is consistent with current castingtechniques. This invention is primarily directed to the method ofproviding coolant, through a blade attachment, to the walls of a4-walled airfoil.

FIG. 13 typifies of the cast turbine blade showing the airfoil section18 and the attachment section 22, the leading edge 24, the trailing edge26 and the pressure side 28. The suction side (shown in FIG. 3) is onthe back side of the pressure side and coextensive therewith. Thisinvention is concerned with the cavities and passages formed in the rootof the blade or the attachment portion 22.

FIGS. 3-10 are sections taken through the attachment portion 22 anddemonstrate the transition extending from the base of the airfoil to thebottom of the attachment 22. As will be appreciated in this embodiment,the core segments 10 include three core sections 32, 34 and 36 of thesuction side hybrid core; two core sections 38 and 40 of the pressureside hybrid core and five core segments of the main body core 42, 44,46, 48 and 50. The inner wall segments as noted by the points 52 and 52Aas they progress toward the root, taper and merge to form a single wallcentered in the blade attachment as can be seen by the planar core joint60 of FIG. 8.

As best seen in FIGS. 9 and 10 which are sections taken along lines 9--9and 10--10 of FIG. 5, the main body core and mold define the inner walls62 and outer walls 64. The configuration of the core segments provide athickened pressure side hybrid core as noted in the area designated by Aand a thickened suction side hybrid core as noted in the area designatedA1. The inner walls 62 line up to form a single wall 66 at location Band form a planar core joint surface extending down to the bottom of theattachment section. As noted, the hybrid ceramic cores are cantileveredoutboard from location B. It will be appreciated that the flashoccurring at core joint at location B is parallel to coolant flow andaccessible for removal.

As mentioned earlier, it is important that the mating surfaces of thesegments of the ceramic core that are bonded together before beingassembled in the mold accurately fit. The fit, in fact, must almost havea zero tolerance. By virtue of this invention as best seen in FIG. 11,which is an enlarged perspective view, the core segments E, F and G areconfigured so that all of the adjacent surfaces of the base portion Hare flat and planar. As noted from this FIG., in the segment F thefingers extending axially from the base portion H of segments E and Gmerge as shown by G₁, G₂ and G₃, and E₁ and E₂ respectively, and alignwith the plane 60. This assures that the tolerances can be held to aminimum. Obviously, this eliminates the need to match contours or haveclose tolerance plug and socket type inserts thereby, eliminating orminimizing the potential mismatching of surfaces.

This core configuration lends itself to being utilized in theconventional lost wax investment casting process for hollow turbineblades.

FIG. 12 exemplifies another embodiment of this invention whereby ametering plate 80 having the metering orifice 82 is attached to the footof the blade. Orifice 82 is sized to selectively meter the flow into theinternal cavities in the blade in order to control the pressure ratioacross the film cooling holes of the blade between the engine's gas pathand the coolant. This assures that a more favorable film coolingeffectiveness is obtained adjacent the exterior surface of the blade.The film cooling holes are best seen in FIG. 13. For additionalinformation about the finished turbine blade reference should be made tothe blades disclosed in the following patents:

U.S. Pat. No. 5,243,759 granted to Brown et al on Sep. 14, 1993 entitled"Method of Casting to Control Air Flow Rate of the Airfoil TrailingEdge" and U.S. Pat. No. 4,820,123 granted to Hall on Apr. 11, 1989entitled "Dirt Removal means for Air Cooled Blades" both of which arecommonly assigned to the assignee of this patent application and U.S.Pat. No. 4,010,531 granted to Andersen et al on Mar. 8, 1977 entitled"Tip Cap Apparatus and Method of Installation". These patents areincorporated herein by reference.

Although this invention has been shown and described with respect todetailed embodiments thereof, it will be appreciated and understood bythose skilled in the art that various changes in form and detail thereofmay be made without departing from the spirit and scope of the claimedinvention.

I claim:
 1. The method of configuring a core used in defining a hollowportion in a cast turbine hollow blade that includes a tip section, aroot section, a leading edge, a trailing edge, a pressure side and asuction side comprising the steps of:forming the core from a ceramicmaterial that is capable of being leached from a mold used in a castingprocess into three independent segments that are complementary toadjacent segments; locating the core in the mold so that it extends fromthe root adjacent to the tip and between the leading edge, trailingedge, pressure side and suction side, fairing and contouring each ofsaid segments from the top of the root section to the bottom of the rootsection so that the independent segments taper and merge to define asingle wall.
 2. The method as claimed in claim 1 wherein the segmentadjacent to the upstream end of the juncture where the segments becomesa single wall relative to the direction of the tip section to the rootsection is thicker relative to the portion downstream of that juncture.3. The method as claimed in claim 1 wherein said thicker-portion isadjacent the pressure side and the suction side.
 4. The method asclaimed in claim 3 wherein said core is formed from three core segmentson said suction side, two core segments on the pressure side and fivecore segments disposed therebetween.